How to import a .mtl file?
When you import the .obj file, Blender will also import the associated .mtl file* by default as well which is usually the same name. The colors applied to the model will show in the viewport and to see the texture(s) (if any), enable texture shading with AltZ.

* This is the file that should be generated alongside the obj when it was created and will hold color data and paths to textures if any. Opening the obj file in a text editor should show this near the top.